1. Field of the Invention
The present invention relates to an optical storing apparatus from/to which information is read and written by a positioning control of a laser beam to a medium track on the basis of a tracking error signal and, more particularly, to an optical storing apparatus for correcting a tracking error signal so that a lead-in and an on-track control of a laser beam to a track center can be properly executed.
2. Description of the Related Arts
Attention is paid to an optical disk as a storing medium serving as a nucleus of multimedia which has rapidly been developing in recent years. For example, as for an MO cartridge of 3.5 inches, MO cartridges of 128 MB, 230 MB, 540 MB, 640 MB, and the like are provided. An optical disk drive using such an MO cartridge is provided as an external storing apparatus of a desktop type personal computer. Further, the use of the optical disk drive is also strongly desired in a notebook sized personal computer having an excellent portability which has rapidly become popular in recent years. In order to equip the optical disk drive as an external storing apparatus as standard equipment, therefore, a miniaturization, a thin size, and further, a low price are requested.
The optical disk drive has a pickup of a linear driving type in the direction which traverses tracks on a medium. The pickup is constructed by a fixed optical system fixed to a casing and a movable optical system which is linearly driven by a VCM. A movable optical unit mounted on a carriage is equipped with a lens actuator and has a relatively complicated mechanism which requires a two-dimensional degree of freedom such that an objective lens is moved in the direction which traverses the tracks by a current supply of a tracking coil and the objective lens is moved in the vertical direction by a current supply of a focusing coil. Such a pickup of the double driving type in which the lens actuator is mounted on the carriage performs a speed control for an acceleration, a constant speed, and a deceleration by the driving of the carriage by the VCM at the time of a seek control (coarse adjustment) for moving a beam toward a target track and executes a lead-in control for leading the beam to the target track by the driving of the lens actuator when the laser beam approaches the target track. After completion of the lead-in to the target track, the beam is allowed to trace the target track by the positioning control of the lens actuator and, simultaneously, to trace a medium eccentricity or the like by the driving of the carriage by the VCM. The structure of the pickup mechanism of the double driving type in which the lens actuator is mounted on the carriage is, however, complicated and the beam positioning control is also complicated since the control of the carriage and that of the lens actuator are combined, so that there are limitations to realize the miniaturization, thin size, and reduction in costs of the pickup. There is, accordingly, a pickup of a single driving type for executing all of the seek control for moving the beam toward the target track, the lead-in control for the target track, and the tracking control for the target track after completion of the lead-in only by the driving of the carriage by the VCM without using the tracking actuator. In the pickup of the single driving type, it is sufficient to mount simple parts such as objective lens, focusing actuator, and the like onto the carriage. Consequently, the carriage can be made small and thin, its mass can be sufficiently reduced, and an inertia occurring by the carriage movement can be reduced as compared with the double driving type in which the lens actuator is mounted, so that a high trace response speed can be obtained. Since it is sufficient to perform only the control by the VCM, there is an advantage such that the seek control, the target track lead-in control, and the tracking control can be also realized by simple control systems and enough reduction in costs can be expected as a result.
On the other hand, in the optical disk drive, arbitrary tracks on the optical disk are accessed at random. In this instance, in order to read information stored on the optical disk or write information to the optical disk, it is necessary to execute a tracking control for accurately positioning the laser beam toward the target track at a high speed. For the purpose of the tracking control, in the optical disk drive, a tracking error signal (hereinbelow, called a xe2x80x9cTESxe2x80x9d) is optically detected. As a method of obtaining the TES in the optical disk apparatus, a push-pull method (far field method) in which detecting sensitivity is high and a signal is obtained by a single beam and an optical system and a circuit are simple is used. The TES signal which is obtained by the push-pull method is a signal obtained by optically and indirectly detecting a deviation (positional error) between the track center and the laser beam by using an interference of the light. The tracking control based on the TES in the single driving type pickup is executed as follows. In the seek control for moving the laser beam to the target track, the speed of the carriage is controlled by the driving of the VCM. That is, a speed control such that a target speed is set in accordance with the number of remaining tracks to the target track and, after acceleration, the target speed is maintained is executed. During the speed control, a down-count such that the number of tracks is obtained by detecting a zero-crossing point of the TES and the number of remaining tracks to the target track is obtained is performed. When the number of remaining tracks to the target track is reduced to a specified value, the control is switched to the deceleration control. When the laser beam approaches a position just before the target track during the deceleration control, the control is switched to a position servo control based on the TES, the VCM is feedback controlled so that the TES is set to zero, and the beam is led to the target track. When the lead-in to the target track is succeeded, an on-track signal is obtained, and the seeking operation is completed. In a state where the laser beam is allowed to trace the track center by the tracking control (on-track control), the reading operation or writing operation from/to the optical disk is permitted. In the tracking control, since the TES is equal to zero at the track center, the carriage is driven by the VCM by the feedback control for always setting the TES to zero. Even if there is a positional fluctuation of the target track due to a disk rotational eccentricity or the like, the laser beam is always allowed to trace the track center.
A desirable position signal having a proportional relation for a physical positional deviation amount (distance) X of a light spot (laser spot) of the laser beam from the track center is set to an ideal TES Zdesired. The TES is, so to speak, merely a signal obtained by performing a modulation by the interference of light to the ideal TES Zdesired. The magnitude of the TES and the actual positional deviation amount X, therefore, do not always have the proportional relation. This is because the positional error is detected as a TES by using the interference of light and is a phenomenon caused by the nature of the TES which is obtained by the push-pull method.
FIG. 1 shows the ideal TES Zdesired and a TES Y for the actual positional deviation amount X with respect to a case where a track pitch TP=1.1 xcexcm. The positional deviation amount X of the axis of abscissa is equal to X=0 at the track center and has a width of xc2x10.55 xcexcm in the lateral direction. As a normalized signal level of an axis of ordinates, a value obtained by converting the level of the TES Y by the track pitch TP=1.1 xcexcm is used. Y=0 at the track center and a range of xc2x10.55 xcexcm in the vertical direction is shown. The relation between the positional deviation amount X and the TES Y in FIG. 1 shows an almost sine wave 300 and can be approximated by, for example, the following equation.
Y=(TP/2xcfx80)sin{(2xcfx80/TP)xc2x7X}
On the other hand, the ideal TES Zdesired having the proportional relation with the actual positional deviation amount X shows a straight line 302 and is obtained by
Zdesired=Kxc2x7X
As will be understood from FIG. 1, the ideal TES Zdesired of the straight line 302 is proportional to the actual positional deviation amount X and linearly changes. On the other hand, although the TES Y which changes like a sine wave 300 almost traces the ideal TES Zdesired of the straight line 302 in a range 306 around a positional deviation amount X=0 serving as a track center as a center, when the TES Y is out of the range 306, it does not trace the ideal TES Zdesired and is saturated.
The lead-in control of the laser beam to the target track is executed by starting the feedback control of a position servo to set the TES to zero when the laser beam reaches a position just before the target track during the deceleration by the speed control. In the lead-in control, as an initial state when the position servo feedback is turned on, if both of the positional error and a relative speed between the laser beam and the target track center are zero, respectively, the lead-in to the track center is certainly succeeded and the control can be shifted to the tracking control. In designing, for example, parameters of the seek control are determined so as to satisfy such optimum initial conditions. In the actual operating state, however, there are often a positional error and a speed error in the initial conditions just before the lead-in by various disturbances due to a vibration, a temperature fluctuation, and the like. In this case, when the beam speed at the start of the lead-in is low, as shown by an arrow 308 in FIG. 1, after the laser beam passed the track center corresponding to a point 304 where the positional deviation amount X=0 the deceleration is sufficiently performed by the feedback control of the VCM according to the magnitude of the TES Y, the TES Y stops in the range 306 where it coincides with the ideal TES Zdesired, and the laser beam can be led to the track center. When the beam speed at the lead-in start time is high, however, as shown by an arrow 310, the TES Y after that the laser beam passed the point 304 corresponding to the track center exceeds the range 306 where the TES Y coincides with the ideal TES Zdesired. The TES Y is deviated into a range where it is saturated. At this position, the feedback amount of the VCM according to the magnitude of the TES Y is insufficient and the deceleration cannot be sufficiently performed, so that the laser beam cannot be returned to the target track center and the lead-in fails. When the lead-in fails, after executing a predetermined error process, it is necessary to perform the seek control again, so that the accessing performance deteriorates. Especially, in the single driving type pick-up, since the lead-in control is executed in the carriage itself, the servo band width of the lead-in control cannot be made sufficiently high, the feedback control of the TES for the carriage speed at the lead-in start time is hard to be effected, a probability of a lead-in failure rises, and it is one of causes of the deterioration of the accessing performance as compared with the lead-in control by the lens actuator of the double driving type pickup.
According to the invention, there is provided an optical storing apparatus in which even if there is a variation in a carriage speed, a lead-in control of a target track to the center can be certainly performed and the number of times of the recovering operation which is caused by a lead-in failure is decreased, thereby reducing an access time.
According to the invention, an optical storing apparatus comprises: a pickup for moving an irradiating position of a laser beam to an arbitrary track position of a medium; an information signal processing unit for reproducing at least information to the medium by the laser beam; a position signal detecting unit for detecting a position signal Y according to a positional deviation amount X in which a track center of the medium is set to 0 on the basis of return light of the laser beam from the medium; a position signal correcting unit for outputting a corrected position signal Z obtained by correcting detection sensitivity characteristics for the positional deviation amount to desired characteristics by performing a correcting arithmetic operation using a predetermined non-linear function on the basis of the position signal Y; and a positioning control unit for performing a tracking control such that the laser beam is moved toward a target track of the medium and the laser beam is lead-in controlled to the center of the target track on the basis of the corrected position signal Z by switching a control mode to a position servo control at a position just before the target track and the laser beam is allowed to trace the target track after completion of the lead-in control. With respect to such an optical storing apparatus, the invention is characterized by comprising a position signal correcting unit for outputting a corrected position signal Z obtained by correcting detection sensitivity characteristics for the positional deviation amount to desired characteristics by performing a correcting arithmetic operation using a predetermined non-linear function to the position signal Y.
According to the position signal correcting unit, in a position range where the sensitivity of the position signal Y detected by the position signal detecting unit deteriorates for an ideal position signal Zdesired to a change of the actual positional deviation amount X of the laser beam, by performing a correcting arithmetic operation using a predetermined non-linear function to the position signal Y, the sensitivity is increased, thereby outputting the corrected position signal Z approximated to the ideal position signal Zdesired. As for the corrected position signal Z corrected by the position signal correcting unit, at the time of the lead-in control of the target track to the track center, an enough feedback amount can be obtained by the correction, even if an initial speed at the start of the lead-in is high, the laser beam can be certainly led to the track center.
When an absolute value |Y| of the position signal Y detected by the position signal detecting unit is equal to or larger than a predetermined threshold value Yth, the position signal correcting unit performs the correcting arithmetic operation using the predetermined non-linear function to the position signal Y, thereby calculating the corrected position signal Z.
When it is assumed that the ideal position signal Zdesired has linear characteristics of
Zdesired=Kxc2x7X
the position signal correcting unit sets an Nth order polynomial such as
Z=aNYN+aNxe2x88x921YNxe2x88x921+ . . . +a2Y2+a1 Y+a0
as a non-linear function which is used for the correction, substitutes the position signal Y for the Nth order polynomial, and calculates the corrected position signal Z. As mentioned above, the ideal TES Zdesired is defined and the corrected position signal Z is obtained from the position signal Y by forming a correcting function for converting so as to approximate to or coincide with the ideal TES. Therefore, the continuity of a gain due to the corrected position signal Z at positions before and after the threshold value Yth is not lost, an excitation of an oscillation due to a fact that the gain is discontinuously switched is not caused, and a stable feedback control can be performed. Specifically speaking, now assuming that the threshold value Yth is set to a predetermined value that is equal to or smaller than a maximum amplitude Ymax of the position signal Y,
I. In a range (|Y|xe2x89xa6Yth) where the absolute value |Y| of the position signal Y is equal to or smaller than the threshold value Yth, the correction position signal Z is calculated by
Z=Y
II. In a range (Yth less than Y) where the position signal Y exceeds the positive threshold value Yth, the corrected position signal Z is calculated by substituting the position signal Y into the following Nth order polynomial.
Z=aNYN+aNxe2x88x921YNxe2x88x921+ . . . +a2Y2+a1Y+a0
III. Further, in a range (Y less than xe2x88x92Yth) where the position signal Y is smaller than the negative threshold value xe2x88x92Yth, the corrected position signal Z is calculated by substituting the position signal Y into the following equation.   Z  =      -          (                                    a            N                    ⁢                                    "LeftBracketingBar"              Y              "RightBracketingBar"                        N                          +                              a                          N              -              1                                ⁢                                    "LeftBracketingBar"              Y              "RightBracketingBar"                                      N              -              1                                      +        ⋯        +                              a            2                    ⁢                                    "LeftBracketingBar"              Y              "RightBracketingBar"                        2                          +                              a            1                    ⁢                      "LeftBracketingBar"            Y            "RightBracketingBar"                          +                  a          0                    )      
Practically, the position signal correcting unit calculates the corrected position signal Z by substituting the position signal Y detected by the position signal detecting unit into the following quadratic polynomial.
Z=xe2x88x92a2Y2+a1Y+a0
That is, in a range (Yth less than Y) where the position signal Y exceeds the positive threshold value Yth, the corrected position signal Z is calculated by substituting the position signal Y into the following quadratic polynomial.
Z=a2Y2+a1Y+a0
In a range (Y less than xe2x88x92Yth) where the position signal Y is smaller than the negative threshold value xe2x88x92Yth, the corrected position signal Z is calculated by substituting the position signal Y into the following expression.
Z=xe2x88x92(a2Y2+a1|Y|+a0)
As an ideal position signal Zdesired, the position signal correcting unit sets the linear characteristics of
Zdesired=KXZxc2x7X
As another ideal position signal Zdesired, in a range (|X|Xth) where an absolute value |X| of the positional deviation amount X is equal to or smaller than a threshold value Xth, the position signal correcting unit sets the linear characteristics of
Zdesired KXZxc2x7X
In a range (Xth less than X) where the positional deviation amount X exceeds the positive threshold value Xth, the unit sets the non-linear characteristics of
Zdesired=KXZxc2x7X+KNL(Xxe2x88x92Xth)n
Further, in a range (X less than xe2x88x92Xth) where the positional deviation amount X is smaller than the negative threshold value xe2x88x92Xth, the unit sets the non-linear characteristics of
Zdesired=xe2x88x92{KXZxc2x7|X|+KNL(=|X|xe2x88x92Xth)n}
and coefficients a2, a1, and a0 of the quadratic polynomial are decided so as to be approximated to the ideal position signal Zdesired of the non-linear characteristics. It is also possible that the non-linear function in the position signal correcting unit is prepared as a table in an RAM (or an ROM) and the correction is performed by referring to the table.
According to a modification of the invention, the position signal correcting unit sets the Nth order monomial equation as a non-linear function and calculates the corrected position signal Z by substituting the position signal Y into the Nth order monomial equation, thereby easily raising the detecting sensitivity. That is, now assuming that the threshold value Yth is equal to a value (Ymaxxc2x7Kth) obtained by multiplying the maximum amplitude Ymax of the position signal Y by a positive coefficient Kth which is equal to or less than 1,
I. In a range (|Y|xe2x89xa6Yth) where the absolute value |Y| of the position signal Y is equal to or smaller than the threshold value Yth, the position signal correcting unit calculates the corrected position signal Z by
Z=Y
II. In a range (Yth less than Y) where the position signal Y exceeds the positive threshold value (Yth, the position signal correcting unit calculates the corrected position signal Z by substituting the position signal Y into the following Nth order monomial equation.
Z=YN/Yth(Nxe2x88x921)
III. In a range (Y less than xe2x88x92Yth) where the position signal Y is smaller than the negative threshold value xe2x88x92Yth, the corrected position signal Z is calculated by substituting the position signal Y into
Z=xe2x88x92|Y|N/Yth(Nxe2x88x921)
Practically, the position signal correcting unit can also calculate the corrected position signal Z in the following manner.
In a range (Yth less than Y) where the position signal Y exceeds the positive threshold value Yth, the corrected position signal Z is calculated by substituting the position signal Y into the following quadratic monomial equation.
Z=Y2/Yth
In a range (Y less than xe2x88x92Yth) where the position signal Y is smaller than the negative threshold value xe2x88x92Yth, the corrected position signal Z is calculated by substituting the position signal Y into the following quadratic monomial equation.
Z=xe2x88x92Y2/Yth
On the other hand, in a range (Yth less than Y) where the position signal Y exceeds the positive threshold value Yth, the corrected position signal Z is calculated by substituting the position signal Y into the following cubic monomial equation.
Z=Y3/Yth2
In a range (Y less than xe2x88x92Yth) where the position signal Y is smaller than the negative threshold value xe2x88x92Yth, the corrected position signal Z is calculated by substituting the position signal Y into the following cubic monomial equation.
Z=xe2x88x92|Y|3/Yth2
In case of the cubic monomial equation, since the position signal Y is positive in a range (Yth less than Y) where the position signal Y exceeds the positive threshold value Yth, the cubic monomial equation becomes
Z=Y3/Yth2
Since the position signal Y is negative in a range (Y less than xe2x88x92Yth) where the position signal Y is smaller than the negative threshold value xe2x88x92Yth, the cubic monomial equation similarly becomes
Z=Y3/Yth2
That is, in case of the cubic monomial equation, since Y3 is an odd monomial equation, even in the case where the position signal Y lies within the range (Yth less than Y) and the case where it lies within the range of (Y less than xe2x88x92Yth), the cubic monomial equations become
Z=Y3/Yth2.
The above and other objects, features, and advantages of the present invention will become more apparent from the following detailed description with reference to the drawings.